In a variety of marine environments, seismic surveys are conducted to gain a better understanding of geological formations beneath a body of water. Marine seismic source arrays are used to generate acoustic pulses in the water, and hydrophones detect the reflected signals. Firing controllers are employed to trigger the firing of the acoustic source elements so the main pressure pulse of each element is synchronized in the farfield in the desired direction. For example, the triggering may be conducted such that the primary pulses of each element coincide in a vertical direction in the farfield. In some applications, the firing controller implements time delays to compensate for individual variations in the mechanical triggering mechanisms of the acoustic source elements. Triggering delays can also be used to compensate for geometric variations of the source array.
One approach to quantifying the mechanical triggering delay is to use a time-break sensor. The time-break sensor is positioned inside or proximate the acoustic source element, e.g. air gun, and a specific attribute is detected in the signal measured by the time-break sensor. For example, the attribute may comprise signal maximum amplitude, time of threshold, zero-crossing, or other suitable attributes. The time delay between sending the firing signal and the time of the detected attribute in the time-break signal is processed via a firing control algorithm to adjust the time of the next firing signal.
However, acoustic source element synchronization using air gun mounted time-break sensors provides only an indirect way of synchronizing the peak pressure of the emitted acoustic signals. The approach assumes a constant, source element independent, time offset between the detected attribute in the time-break signal and the time of peak acoustic pressure. In many applications, this assumption is not valid and the time-break synchronization results in sub-optimal alignment of peak acoustic pressure of the acoustic signals. Sometimes, the problem may be mitigated by using firing controllers that support tuning and measurements from nearfield hydrophones. However, with modern compact array configurations it is not possible to distinguish acoustic signals from adjacent air guns in the unprocessed nearfield hydrophone measurements.
Another problem with conventional air guns is the emission of significant acoustic amplitude outside of the frequency range of interest for seismic exploration. The out of band signal represents noise that can interfere with measurements and/or have an adverse affect on marine life.